Position-measuring devices are used in automation technology, for example, to determine, in closed-loop drive mechanisms, actual position values which are needed by subsequent electronics, such as a numerical control system, to calculate setpoints for control circuits used to control the drive mechanism (e.g., the feed rate of a tool or workpiece). If the position-measuring devices are in the form of rotary encoders or angle-measuring devices, then these are directly or indirectly coupled, for example, to the shaft of a motor for this purpose. Length-measuring devices measure, for example, linear movements between a machine bed and a machine part that is positionable relative to the machine bed, such as, for example, a movable tool carriage.
Today, absolute position-measuring devices are preferably used. Such devices generate absolute measurement values, which are transmitted from the position-measuring device to the subsequent electronics via digital, mostly serial data interfaces. The measurement values are mostly position values (angular values or linear positions), but there are also known position-measuring devices which deliver velocity or acceleration values; i.e., measurement values which indicate changes in positions over time.
A not-to-be underestimated cost factor in the connection of position-measuring devices to subsequent electronics, such as, for example, a machine tool controller, is the number of electrical lines required for operation, because these largely determine the cost of the high-quality data cables used. For this reason, there is an increasing tendency to use serial interfaces where data transmission takes place over only one bidirectionally operated data channel. A serial interface of this type is described in WO 2009/149966 A1. In order to be able to span the sometimes large distances between the position-measuring device and the subsequent electronics, the signals are transmitted differentially over the main transmission path according to the known RS-485 standard. In this case, a single line pair is necessary for the signal transmission. The position-measuring device and the subsequent electronics are each provided with suitable line drivers and receivers for transmitting and receiving data.
In order to reverse the data direction, the transmitting device deactivates its line drivers after the data frame to be transmitted has been completely output. Once the receiving device has received the complete data frame, it activates its line drivers and thus becomes the transmitting device, which in turn outputs a data frame to the data channel. During the period of time between the deactivation of the line drivers at one end and the activation of the line drivers at the other end, the line pair is high-ohmic with respect to the connected devices and is therefore susceptible to spurious signals. This time period is largely dependent on the propagation delay of the signals on the lines. In order for the receiving device to reliably detect the beginning of a data transmission after a reversal of the data direction, the transmitting device always first transmits a start sequence (preamble). However, it has been found that, under unfavorable conditions, signal reflections can lead to misidentification of a start sequence.
Signal reflections occur when the data transmission link is not optimally terminated; i.e., when a terminating resistance at the receiver end is not optimally matched to a line impedance of the line pair used for the data channel. In this case, signals arriving at the receiver are reflected and returned to the transmitter. When the data direction is reversed, the original transmitter becomes the receiver of the reflected signals. If these signals happen to contain the bit pattern of the start sequence, this may be misinterpreted as the beginning of a data transmission.
This problem can be circumvented by not activating the line receiver of the receiving unit until the signal reflections resulting from a data transmission have reliably decayed. However, this results in undesired lengthening of the access cycles and, therefore, is to be avoided.